Cell in its environment Coulter

Diffusion  The cell membrane is selectively permeable, which means that some substances can pass through the membrane while others cannot.  Diffusion- is the process by which molecules move from an area of higher concentration to an area of lower concentration.  Concentration- is the amount of a substance in a given volume.

What causes diffusion?  Molecules are always moving. As they move they bump into each other.  The more molecules in an area the more they will collide.  Collisions cause molecules to move away from each other.  The molecules will continue to spread out until they are eventually evenly spread out throughout the area.

Diffusion of oxygen  There is higher concentration of oxygen molecules in water than inside the cell.  The cell membrane is permeable to oxygen molecules.  Oxygen molecules diffuse from the higher concentration to lower concentration

Osmosis!!  Osmosis- is the diffusion of water molecules through a selectively permeable membrane  Because cells cannot function properly without adequate water, many cellular processes depend on osmosis.

Osmosis and diffusion  In osmosis water molecules move by diffusion from an area where they are highly concentrated through the cell membrane to an area where they are less concentrated.

Effects of osmosis on plants  Plant cells are enclosed by a rigid cell wall.  When the plant cell is placed in a hypotonic solution, it takes up water by osmosis and starts to swell, but the cell wall prevents it from bursting.  The plant cell is said to have become "turgid" i.e. swollen and hard.  The pressure inside the cell rises until this internal pressure is equal to the pressure outside.  This liquid or hydrostatic pressure called the turgor pressure prevents further net intake of water.  Turgidity is very important to plants as it helps in the maintenance of rigidity and stability of plant tissue and as each cell exerts a turgor pressure on its neighbor adding up to plant tissue tension which allows the green parts of the plant to "stand up" into the sunlight.

Effects of osmosis in animals  Animal cells do not have cell walls. In hypotonic solutions, animal cells swell up and explode as they cannot become turgid because there is no cell wall to prevent the cell from bursting.  When the cell is in danger of bursting, organelles called contractile vacuoles will pump water out of the cell to prevent this.  In hypertonic solutions, water diffuses out of the cell due to osmosis and the cell shrinks.  Thus, the animal cell has always to be surrounded by an isotonic solution.  In the human body, the kidneys provide the necessary regulatory mechanism for the blood plasma and the concentration of water and salt removed from the blood by the kidneys is controlled by a part of the brain called the hypothalamus.

Effects of osmosis

Active transport  Active transport describes what happens when a cell uses energy to transport something.  Active transport usually happens across the cell membrane. There are thousands of proteins embedded in the cell's lipid bilayer.  Those proteins do much of the work in active transport.  They are positioned to cross the membrane so one part is on the inside of the cell and one part is on the outside.  Only when they cross the bilayer are they able to move molecules and ions in and out of the cell.  The membrane proteins are very specific.membrane proteins  One protein that moves glucose will not move calcium (Ca) ions.  There are hundreds of types of these membrane proteins in the many cells of your body.

Active transport  Many times, proteins have to work against a concentration gradient. That term means they are pumping something (usually ions) from areas of lower to higher concentration.